1. Field of the Invention
The present invention relates to safety joints used in pressure containing tubular system, such as pipelines, flow-lines, offshore drilling risers and the like where it is desirable to have a pre-selected "weak link" so that if an unexpected, substantial axial load is applied to the tubular system, such as through inadvertent hooking of a drag line or the like, the tubular system will separate at a pre-selected point to minimize damage. Particularly, the present invention relates to multiple tubes having a shared, pre-selected "weak link".
2. Description of the Prior Art
In the petroleum and related industries, it is frequently desirable to drill and complete a multiplicity of individual wells in an underwater area too large or, for other reasons, too difficult to be accomplished from a single structure or platform.
The production from these individual underwater wells (referred to as satelite wells) is frequently conducted to a central gathering point located on a platform or shore installation via pipeline or multiple pipe lines.
Each satelite well may produce from a multiplicity of production zones, and each of the production zones is generally produced separately and its production kept isolated one from another due to pressure differences, and other dissimilarities. This isolation of dissimiliar production fluids must generally be continued to and beyond the central gathering point.
In addition to one flow line for each production zone in a particular satelite well, an additional line is required if the operator wishes to service these wells remotely by employing a technique known as T.F.L. (through-flow-line).
This technique is a means by which various service tools are pumped through one of the flow lines from the central gathering point to and down into an individual tubing string within a given satelite well.
The fluids displaced by this pumping operation are returned to the central gathering point through an additional flow line. This return line is subsequently employed to reverse the pumping flow and return the service tools to the central gathering point.
As an example: a dual completion (a well producing from two (2) zones) requiring T.F.L. services would require one flow line for each of the two production zones plus an additional flow line to accomodate T.F.L. service; or a total of three (3) flow lines. These three flow lines would be continuous from a given satelite well to the central gathering points.
For reasons of economics, ease of installation, underwater identification and strength considerations, operators frequently fasten or bundle flow lines into a single unit. This "bundled" unit proceeds from a particular satelite well to its destination at a central gathering point.
Because satelite well heads and central gathering points represent points of relative fixity, it is desirable to have the relatively flexible flow line bundle automatically disengage itself from the fixed points should the flow line bundle experience severe physical loads; e.g., dragging boat anchors shifts in the ocean bottom or substantial movement occasioned by ocean currents, storms and the like.
In normal application without a weak link, if such a branch line were hooked and pulled excessively, the trunk line would likely be buckled or damaged in similar manner. By having a separate weak link placed in the branch line next to the trunk line, the branch line would simply break away without damage to the trunk line of any substantial amount.
Another situation where it might be desirable to have a weak point in an underwater pipeline would be at the bottom of the riser which is fastened to an offshore production platform. Occasionally, a pipeline will be pulled by an anchor being hooked onto it or by loads exerted by storm conditions. If the pipeline connected to the bottom of the riser is not protected by a weak joint, then the riser pipe might actually be pulled away from the platform, causing damage to the riser in the form of a buckle or rupture.
For examples of prior art attempts to provide weak links in pipeline systems having a single tubing member, see U.S. Pat. No. 4,059,288, issued Nov. 22, 1977, to Harvey O. Mohr, entitled "Pressure Balanced Safety Pipeline Connector"; U.S. Pat. No. 4,348,039, issued Sept. 7, 1982, to Jack E. Miller, entitled "Release Coupling"; the brochure of Big-Inch Marine Systems, Inc., entitled "Big-Inch Marine Systems, Inc., Operating Of The Load Limiting Connector"; and the brochure of Cameron Iron Works, Inc., entitled "Breakaway Flowline Coupling". The Cameron and Big-Inch prior art devices, to the knowledge of the inventor, require the collet to bear both the physical and the hydraulic load. Mohr doesn't disclose a collet.
Accordingly, it is an object of the present invention to provide a multiple tube safety joint where the tubes have different flowing pressures.
It is a further object of the present invention to use pipeline pressure for relieving pre-load used for lowering a multiple tube pipeline onto the ocean floor.
It is a further object of the present invention to relieve the multiple tube connector from bearing physical load and have it bear only hydraulic load.
It is a further object of the present invention to provide a pressure compensated multiple safety joint that permits separation of the collet without substantial load on the collet in the event the tubular system is subjected to loads in excess of those deemed operational but less than loads which would overstress or damage the tubular or adjacent system. In this manner, the safety joint protects adjacent equipment, physically attached to the tubular system, such as valves, larger pipelines, wellheads, blowout preventer stacks, pressure vessels and the like from being damaged under physical load.
It is also an object of the present invention to provide a hydromechanical lock-out feature for preloading the mechanical tension on a multiple tube safety joint, with remote disengagement.
Therefore, it should be recognized that a connector capable of disengaging at a pre-detemined load to be installed at one or both ends of the flowline bundle would be desirable.
It might further be appreciated that connectors commonly employed by the industry such as flanges, clamp type or threaded connections are subject to variations in conduit pressure loads and therefore are unpredictable in tensile strength at any given time. These standard type connectors (flanged, clamp) are subject to additional forces when submerged in water, rendering the establishment of a predictable separation load in the prior art difficult within a reasonably acceptable range under usual conditions.